Recent studies have suggested an important role for growth factors and transcription factors in the initiation and subsequent development of teeth. BMP and FGF may function as morphogens in these processes. In the avian oral cavity there exists a residual tooth germ resembling the tooth rudiment arrested at the lamina stage in reptiles and mammals. This arrest of development at the initiation stage in Aves may be due to the failed interactions between the presumptive dental epithelium and mesenchyme. Thus, the avian presumptive tooth germ provides an excellent model for studying the early events, and the molecular basis, of epithelial-mesenchymal interactions during vertebrate organogenesis. Based on previous studies, a hypothesis is proposed that once tooth initiation site is determined, Bmp4 expression in the vertebrate presumptive tooth germ is required for dental epithelial invagination. Activation of BMP signaling pathways in the dental epithelium is critical for the initiation and subsequent development of vertebrate tooth. A three-step model for the role of BMP in tooth initiation and early development is proposed. The hypothesis and model proposed will be rigorously tested in both mouse and chick by several complementary approaches in this grant application. Aim 1 is designed to reactivate the early chick odontogenic signaling pathway (BMP and FGF signaling pathways) and to further enhance the development of the chick tooth germ in vitro and in ovo by a combination of several techniques such as retroviral infection, organ culture, chick CAM culture and mouse intraocular and kidney capsule implantation. Aim 2 will focus on the role of BMP in the initiation of tooth development by in vitro organ culture and tissue recombination with BMP blocking antibodies. In aim 3, the importance of BMP signaling pathways in vertebrate tooth initiation will be examined by ectopic expression of constitutively activated BMP receptors in the chick tooth germ in ovo and by forced expression of dominant negative BMP receptors in the chick dental epithelium before recombined with chick skin mesenchyme through retroviral infection. Lastly in aim 4, we will test the ability of ectopically expressed Bmp4 to rescue the Msx1 mutant tooth phenotype which exhibits an early arrest of tooth development and the bud stage in mice. Our long-term goal is to substantiate our understanding of the molecular basis of epithelial-mesenchmal interactions during vertebrate organ formation. In addition, understanding the molecular basis of tooth initiation may provide insight for the study of human tooth regeneration and the many other developmental processes driven by epithelial-mesenchymal interactions.